The invention relates to a powered land vehicle including at least one spring-applied hydraulic release (SAHR) vehicle brake.
In such a brake first and second brake elements, mounted respectively on a rotatable part of the drive train of the vehicle such as a rotatable wheel on the one hand and on a non-rotatable part such as a suspension component or the vehicle frame on the other are moveable between mutually engaged and separated configurations.
In the separated configuration the rotatable component is free to rotate, thereby permitting the transmission of drive from e.g. a diesel engine forming part of the vehicle to a ground-engaging member such as a wheel including a tyre. In the mutually engaged configuration the second element brakes rotation of the rotatable component supporting the first brake element with the result that the transmission of drive is prevented and therefore the vehicle as a whole is braked.
In almost all cases the second brake element is moveable towards and away from the first brake element. A resiliently deformable member such as a coil spring acts on the second element or on a member connected thereto in order to bias it into engagement with the first brake element.
The second brake element includes or is connected to a piston in a chamber having a connection to a hydraulic control circuit that is capable of applying pressure to the piston and hence indirectly to the second brake element itself. The hydraulic control circuit is selectively capable of being energised in order to assure separation of the first and second brake elements under normal circumstances. As a result the rotatable element is maintained able to rotate most of the time.
When the vehicle operator selects the brake function however control elements connected to the hydraulic control circuit cause venting of the fluid pressure acting on the piston. This in turn permits the resiliently deformable spring to urge the second brake element into engagement with the first, whereby application of the SAHR brake occurs in order to brake the vehicle regardless of the output of the engine or operative status of a variable-ratio transmission forming part of the drive train of the vehicle.
Thus the SAHR vehicle brake serves primarily as a park brake that is intended reliably to brake the vehicle when it is stationary, regardless of whether the engine of the vehicle is running. The design of the SAHR lends itself to this use because the presence of the resiliently deformable spring maintains the vehicle in a braked condition even when the engine is switched off.
SAHR vehicle brakes typically are fitted in large, powered land vehicles such as tractors, combine harvesters, forage harvesters and other harvesting machines, excavators, bulldozers, utility vehicles such as farm loaders and multipurpose farm vehicles, mobile cranes, and similar vehicles that while designed to perform specialised tasks usually in off-road situations nonetheless are intended to travel on roads between work sites. SAHR vehicle brakes are required in such vehicles to provide for operator and third party safety in both on-road and off-road situations. The invention is of utility in all vehicles of the general kind listed above, together with numerous further vehicle types.
As indicated the braking systems of large vehicles of the general category indicated typically are electrohydraulic and therefore consist of braking elements the mutual engagement and separation of which is under the control of one or more hydraulic circuits. The hydraulic circuits include circuit components such as valves the operating states of which are alterable between two or more configurations through the action of electrically powered elements such as solenoids that move moveable parts of the valves from one position to another.
SAHR vehicle brakes by reason of the permanent tendency of the spring to urge the brake elements into engagement include failure safety in their designs. A drawback of this however is that in the event of a sudden failure of the electrical system forming part of the electrohydraulic braking arrangement the SAHR brake can be applied very rapidly and without warning. If such a scenario arises while the vehicle is moving it can lead to vehicle accidents that in view of the large sizes of the vehicles in which SAHR vehicle brakes are installed have the potential to be very serious.
Thus there is a need for failure safety in SAHR vehicle brakes such that in the event of e.g. an electrical failure involving the control elements of the electrohydraulic circuit while the vehicle is moving (or while the vehicle is moving at a speed that is greater than a threshold level) the brake does not automatically apply as outlined above.
Equally importantly the failure of some components of an SAHR brake can lead to other potentially dangerous situations. As an example of a value that normally is responsible for renting the hold—off fluid pressure acting on the piston becomes struck in a non-venting position activation of SAHR brake would not happen as demanded by the vehicle driver and/or the control systems of the vehicle.
There is a further need that a vehicle operator should remain capable of applying the SAHR brake in order to meet a park braking requirement at zero or a chosen low vehicle speed even if there has been an electrical failure involving the control elements of the circuit.
It is furthermore desirable for the SAHR brake to be operable even when the vehicle ignition is switched off; and for the SAHR brake under non-fault conditions to be capable of automatically releasing when the vehicle starts to move following a period of non-movement. When the SAHR brake is applied in a stationary vehicle/parking situation the braking effort should be sufficient to assure non-movement of the vehicle at least on an incline the gradient of which is less than a predetermined threshold.
Yet a further requirement in the art is for an SAHR brake that can be applied safely, i.e. progressively under operator control, and not suddenly, in an emergency braking situation.
The invention seeks to solve or at least ameliorate one or more problems encountered in prior art SAHR brakes.